Traditionally, pharmaceutical and therapeutic actives can be administered to the body by a number of routes, including ingestion, injection, inhalation, and topical application. Absorption of an active by ingestion, injection, or inhalation generally gives systemic distribution of the active throughout the body. Systemic distribution of the active may be unsatisfactory for three reasons. First, these modes of administration produce non-specific distribution. The active is distributed through the entire body and not localized. Second, there may be undesirable effects such as toxic or irritating reactions on non-target organs or regions. Finally, to achieve the desired effect at the target organ or region, a higher dosage than might otherwise be desired must be administered to compensate for systemic dilution of the active.
In contrast to systemic delivery, topical delivery is application of an active in a manner so that it acts primarily at the site of application. The above-described deficiencies of systemic delivery are not encountered when an active is applied topically. Rather, topical application affords the opportunity to minimize the dosage and confine the active to the region of the body to which it is applied. Thus, systemic distribution of the active throughout the body is obviated. Typical sites of topical delivery include application to the dermal, ophthalmic, and mucous membranes and tissues, such as the hair, skin, eyes, ears, mouth, nose, throat, rectum, vagina, and urethra.
However, despite these advantages of topical delivery, most current topical delivery formulations are inefficient and therefore have limited utility. There are three reasons for this inefficiency of current topical delivery technology. First, skin and mucous membranes possess good barrier properties and the permeability of most actives through these barriers generally is poor. Second, actives applied topically are subject to migration and loss due to perspiration, natural tissue lavation, and mechanical action particularly because such actives are not substantive, not readily absorbed by the skin, and do not form films. Third, because most pharmaceutical or therapeutic actives are relatively simple, low molecular weight compounds or mixtures, these actives have limited solubility in common solvents such as water and alcohol. The actives tend to crystallize and flake-off the skin, for example, before they can be absorbed.
Consequently, considerable effort has been and is being expended in search of a proper delivery system which can minimize undesirable crystallization of the active, deliver the active to the application site, control the dosage thereof, and optimize its availability in its active form. Most known topical delivery systems are petrolatum-based cremes and ointments. These unctuous formulations are unsatisfactory because they are at best uncomfortable and messy when applied to skin and mucous membrane (mucosa).
A topical delivery system cannot be considered fully satisfactory if it is deficient with regard to any of the above-described criteria. For example, a delivery system which does not ensure that the active efficiently penetrates the application site is not satisfactory because it requires that an excess of active be incorporated into the delivery system to ensure delivery of an effective quantity. The remaining active, i.e., that which does not penetrate the application site, is wasted. Similarly, active which is allowed to migrate from the application site, or to crystallize before it penetrates the site, is wasted. Further, a delivery system which satisfies each criterion will be adjudged a failure by a consumer who is dissatisfied because the delivery system leaves an unpleasant residue. For example, an unctuous residue, which is unpleasant to the touch and messy, may cause a consumer not to utilize the treatment. Thus, such delivery systems are unsatisfactory.